foilantennas

8/8/2019 foilantennas

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ANTENNAS FOR LOW POWER APPLICATIONSBy Kent Smith

Introduction:

There seems to be little information on compact antenna design for the low power wireless field. Goodantenna design is required to realize good range performance. A good antenna requires it to be the right

type for the application. It also must be matched and tuned to the transmitter and receiver. To get the best

results, a designer should have an idea about how the antenna works, and what the important design

considerations are. This paper should help to achieve effective antenna design.

Some Terms:

Wavelength; Important for determination of antenna length, this is the distance that the radio

wave travels during one complete cycle of the wave. This length is inversely proportional

to the frequency and may be calculated by: wavelength (cm)=30000 / frequency (Mhz).

Groundplane; A solid conductive area that is an important part of RF design techniques. Theseare usually used in transmitter and receiver circuits. An example is where most of the traces

will be routed on the topside of the board, and the bottom will be a mostly solid copper area.

The groundplane helps to reduce stray reactances and radiation. Of course, the antenna line

needs to run away from the groundplane.

dB, or decibel; A logarithmic scale used to show power gain or loss in an rf circuit. +3 dB is twice

the power, while -3 dB is one half. It takes 6 dB to double or halve the radiating distance,

due to the inverse square law.

The Basic Antenna, and how it works.

An antenna can be defined as any wire, or conductor, that carries a pulsing or alternating current. Such a

current will generate an electro-magnetic field around the wire and that field will pulse and vary as the

electric current does. If another wire is placed nearby, the electro-magnetic field lines that cross this wire

will induce an electric current that is a copy of the original current, only weaker. If the wire is relativily

long, in terms of wavelength, it will radiate much of that field over long distances.

The simplest antenna is the Whip. This is a quarter

wavelength wire that stands above a groundplane. The

most common examples are found on automobiles and

are used for broadcast radio, CB and amateur radio, and

even for cellular phones. This design goes back to the

1890's when Marconi set out to prove that radio signals

could travel long distances. To be successful, he had tostretch a long wire above the ground. Due to the low

frequencies, thus a long wavelength, the wire had to be

long. He also found that the wire worked better when it

was high above ground.

1/4

wavelength

Basic

Full-size

Whip:

Im Gree 79, CH-8566 Ellighausen, Fon +41(71)698 6480, Fax +41(71)698 6481, e-mail: [email protected], www.wirelessworldag.com


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All antennas, like any electronic component, have at least two connection points. In the case of the whip,

there must be a connection to a ground, even if the groundplane area is nothing more than circuit traces

and a battery. The whip and groundplane combine to form a complete circuit. The electro-magnetic field

is set up between the whip and the ground plane, with current flowing through the field, thus completing

the circuit. Ideally, a groundplane should spread out at least a quarter wavelength, or more, around the

base of the whip. The groundplane can be made smaller, but it will affect the performance of the whip

antenna. The groundplane area must be considered when designing an antenna.

A quarter-wave whip is not a compact antenna. At 1 Mhz, in the AM Broadcast band, one quarter of the

wavelength is about 246 feet, or 75 meters. At 100 Mhz, in the FM Broadcast Band, it is nearly 30 inches

(75 cm). This dimension continues to shrink at higher frequencies, being nearly 3 inches (7.5 cm) at 1000

Mhz. A simple formula for the quarter-wave (in cm) is: 7500 divided by the freq. (in Mhz), or for inches:

2952 / freq. (in Mhz). This formula is only a starting point since the length may actually be shorter if: the

whip is overly thick or wide, has any kind of coating, or is not fed close to ground. It may need to be

longer if the ground plane is too small.

The length of the antenna should be measured from the point where it leaves close proximity to ground,

or from the transmitter output. If a whip is mounted on a box, and connected to the transmitter with plain

wire, that wire becomes part of the antenna! To avoid mistuning the antenna, coaxial cable should be used

to connect to an external antenna. On a circuit board, the equivalent to coax is a trace that runs over agroundplane (groundplane on the backside). The above are examples of transmission lines, whose purpose

is to efficiently transfer power from one place to another with minimum loss. Do not try to run an antenna

line too close to ground, it becomes more of a transmission line than an antenna. Fortunately for those

who need a small remote device, a transmission line left open-ended will radiate some energy.

Antenna Characteristics:

Gain:

An antenna that radiates poorly has low gain. Antenna gain is a measure of how strongly the antenna

radiates compared to a reference antenna, such as a dipole. A dipole is similar to a whip, but the

groundplane is replaced with another quarter-wave wire. Overall performance is about the same. An

antenna that is 6 dB less than a dipole is -6 dBd. This antenna would offer one half the range, or distance,

of the dipole. Compact antennas are often less efficient than a dipole, and therefore, tend to have negative

gain.

Radiation Pattern:

Radiation is maximum when broadside, or perpendicular to a wire, so a vertical whip is ideal for

communication in any direction except straight up. The radiation pattern, perpendicular to the whip,

can be described as omni-directional. There is a "null", or signal minimum, at the end of the whip. With a

less than ideal antenna, such as a bent or tilted whip, this null may move and partly disappear. It is

important to know the radiation pattern of the antenna, in order to insure that a null is not present in the

desired direction of communication.

Polarization:

It is important that other antennas in the same communication system be oriented in the same way, that is,have the same polarization. A horizontally polarized antenna will not usually communicate very

effectively with a vertical whip. In the real environment, metal objects and the ground will cause

reflections, and may cause both horizontal and vertical polarized signals to be present.

Impedance:

Another important consideration is how well a transmitter can put power into an antenna. If a transmitter

or receiver is designed for a 50 Ohm load, the antenna should have an impedance near 50 Ohms for best

results. A whip over a flat groundplane has an impedance near 35 Ohms, which is close enough. The

impedance changes if the whip is mistuned or bent down, or if a hand or other object is placed close to it.



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The impedance becomes lower as the antenna is bent closer to ground. When the whip is tilted 45 degrees,

the impedance is less than 20 Ohms. When the whip is bent horizontal to one-tenth of a wavelength above

ground, the impedance approaches 10 Ohms. The resulting impedance mismatch, a 5:1 ratio (VSWR) will

contribute an additional loss of 2.6 dB.

Printed Circuit Whip, or Stub

The whip can be made as a trace on a PC Board. This is very practical at frequencies over 800 Mhz. At

lower frequencies, a full size whip may be too long, even when wrapped around a few corners. The length

of the whip may be 10 to 20% shorter than calculation, depending on the dielectric and the thickness of

the board. In most cases, 15% shorter is close enough. If the unit is to be handheld, the antenna can be

made a little shorter, to compensate for the effect of the hand.

At 916 Mhz, a trace that is 2.25 inches (57mm) long

will provide a reasonable impedance when hand

effects are included. Keep the antenna trace away

from other circuitry and ground, a quarter of an inch

(6mm), or more. Non-ground circuit traces may be

seen by the antenna as part of the ground system,and RF voltages can be induced on nearby traces.

Our sample PC Stub is shown in the drawing at

right. The overall size of the board and ground is not

critical. The radiation pattern is omnidirectional,

with a gain of -8 to -12 dBd, when the board is

horizontal. Polarization is horizontal. If the whip did

not run parallel to ground, the gain would be higher,

however, two sharp nulls would be present. If the

board were oriented vertically, with the antenna

above the groundplane, the polarization would be

vertical. The antenna would have an omnidirectional

pattern with -8 dBd of gain.

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0 dB

-20 dB

Radiation Pattern of Open Stub Antenna

(916.5 MHz)

Printed Open Stub:

916.5 MHz

50mm

12

43

70

xFeedpoint

of antenna

circuit area



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Compact Antennas:

The Short Whip

A simple alternative to the whip is to cut it short and add an

inductor near the base of the whip to compensate for the high

capacitive reactance. The inductor can be made by coiling uppart of the whip itself. This type of antenna can have

performance nearly equal to that of a full size whip.

RFM uses such a design for the wire antennas that are

supplied with our demonstration boards. Details of the design

can be found in the HX/RX portion of the Product Data Book.

The RFM short whip is optimized for under-sized

groundplanes. When tested on the edge of a small board, gain

was only 3 to 4 dB less than a full sized whip and

groundplane.

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0 dB

-10 dB

0 dB

-10 dB

-20 dB

83mm

53

43

RFM Whip:on small

groundplane.

433.9 MHz

Loaded Whip Antenna (434 MHz)



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The Short Printed Stub

One big advantage for the short whip is that it can be a

trace on a printed circuit board, with a chip inductor for

matching. If the trace runs parallel to ground, theimpedance will be low, approximately 10 Ohms. In a

handheld unit, the impedance will be raised substantially

through hand effects. For a tenth wavelength strip on a

board with hand effects included, the antenna has a

capacitive reactance of about 150 Ohms. At 433.9 Mhz,

this would require a 56 nH inductor to match the 2.7 inch

(70mm) long line.

The radiation pattern will be fairly omnidirectional, with a

shallow null along one axis. The polarization is roughly

parallel with the edge of the board. Tuning is not extremely

critical, small variations in inductor value or antenna

length will not have a great effect on performance. Oursample designs, at 433.9 and 916 MHz, resulted in

maximum gains of between -12.5 to -14 dBd off the side of

the board. The null dipped down to about -26 dBd. This is

more omnidirectional than some other designs, and hand

effects will help to reduce the null depth.

The key to this design is to keep resistive losses low, use

wide traces (if a pcb trace), and good quality inductors.

Adjust the inductor value for maximum output in the

environment that it will be used. Gain can be improved by

making the whip longer and thus reducing inductance. But,

in some cases, it may be better to shorten the trace and add

inductance rather than to run the antenna close to other

circuit board traces.

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Short Stub (916 MHz)

Short Stub: 433.9 MHz

50mm

37mm

47 nH

25

x

0 dB

-10dB

-20 dB

x

26mm

38

9

27 nH

Short Stub: 916.5 MHz



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The Spiral

Another way to shorten a whip is to coil it up to form a

flattened coil of wire. It can be a trace printed onto a

circuit board. On a board, the length of the trace is a little

shorter than a quarter wavelength. The antenna must nothave a groundplane directly under it, and should occupy

a clear end of the board. For example, start with a six

inch long thin trace wrapped in a 0.75 inch (19mm)

square area, then trim a little of the length until it is

tuned to 433.9 Mhz.

Antenna gain and impedance will vary with the size of

the groundplane. Our 433.9 Mhz version had a fairly

small groundplane area of 17 sq. cm, while the 916

version had a quarter-wave long ground. The 433.9 MHz

antenna had a maximum gain of -10.5 dBd, with a small

null of -24 dBd. The 916 antenna had a gain of -5 dBd

max. Comparable gain is also seen when looking at theboard face-on.

This antenna does not give circular polarization; the

polarization is parallel to the long edge of the board. As

with a stub, when the board is oriented vertically, it is

vertically polarized and omnidirectional. This antenna is

more easily detuned by a nearby hand, which makes it

less suitable for handheld remotes.

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0 dB

-10 dB

-30 dB

Spiral Antenna (434 MHz)

Spiral: 433.9 MHz

x

19mm

70mm

40mm

22 Ga. wire

or equivalent

trace width

feedline

under

board

x

14

70mm

25

Spiral: 916.5 MHz



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The Helical (Coil)

This is similar to a Spiral that is not flattened. Start with

a piece of wire that is 2 or 3 times longer than a whip

and wind it into a coil. The number of turns on the coil

will depend on wire size, coil diameter, and turn spacing.The coil will need to be cut to frequency, and can be fine

tuned by spreading or compressing the length of the coil.

If the coil is wound tightly enough, it may be shorter than

one-tenth of a wavelength. This antenna tunes sharply,

requiring care in tuning. The impedance is less than

twenty Ohms, and depends on the size of the coil and

orientation to ground.

For 433.9 Mhz, we wound 14 turns of 22 gauge wire

around a 0.25 inch (6mm) form. When tuned, its length

was just under one inch. The proximity of this coil to

ground makes a big difference in performance. When the

coil runs near and parallel to ground, maximum gain isonly -18 dBd. When the loose end of the coil was pulled

away from ground, as shown in the alternate version

drawing, gain increased to -5.5 dBd, and the null became

deeper.

The big problem with this antenna is the mechanical

construction and it's bulky size. It can be easily de-tuned

by nearby objects, including a hand, so it may not be

good for handheld use.

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-10 dB

-20 dB

0 dB

Helical Antenna (434 MHz)

Helical:

433.9 MHz

38mm

50mm

x

6

stretch

to tune

x

Alternate

Version of

Helical Ant.

433.9 MHz



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Chip Antenna

The latest entry into the antenna field is the tiny chip antenna.

They are surface mount devices that are typically 8 by 5 by 2.5

mm, making them the smallest design available. They may be

found for frequencies less than 300 Mhz and up to 2500 Mhz.These antennas are similar to whips in behavior, only much

smaller. If an antenna can be reduced in size, while maintaining

efficiency, bandwidth will be reduced. So these devices have a

very narrow bandwidth and must be made to the exact frequency.

These devices are very groundplane dependant. As a result, they

are easily detuned by hand effects, the wrong size groundplane, or

even the wrong thickness and dielectric of the board. The chip

antenna must be used according to the manufacturers

recommendations.

For 433.9 Mhz, we mounted a chip on a 5 inch long board and

obtained a maximum gain of -10 dBd. Not bad when you considerthat the spiral has equal gain, but consumes five times as much

area on the board. The 916 version did better with a 2.6 inch long

groundplane for a maximum gain of -3.2 dBd. The polarization is

parallel to the long axis of the chip, so maximum radiation is

perpendicular to the long axis. There is a deep null (nearly 40

dB!) looking at each end of the chip. This would be a big problem

if an omni-directional pattern is required from a horizontal circuit

board. When the board is vertical, the pattern is omnidirectional.

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-10 dB

-20 dB

-40 dB

0 dB

Chip Antenna (434 MHz)

11

127mm

34mm

x

Chip Ant.: 433.9 MHz



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The Loop

The loop is entirely different from a whip, in that both

ends of the antenna are terminated. In this case, the end

that is opposite the transmitter (or receiver) is grounded.One advantage is that a capacitor can now be used to

tune and match the antenna, instead of a coil. Another

advantage is that the loop is not easily detuned by hand

effects, although the impedance may still vary. The loop

can be made small, is not groundplane dependant, and

requires no more space than a short whip. For these

reasons, loops are very common in handheld devices.

There are some disadvantages. Small loop antennas have

a reputation for poor gain. A small loop will have a very

narrow bandwidth. This makes tuning extremely critical.

Tuning is often done with a variable capacitor, which

adds to the cost, both parts and labor. If the loop is largeenough, it may be practical to use a non-variable

capacitor. This requires careful adjustment in

engineering stages, to ensure that it is properly tuned

with a standard value capacitor.

Our example loop antenna covers a 12 by 35mm area on

the end of a board. It is tuned to 433.9 Mhz with a

variable capacitor. This antenna is very omnidirectional,

but had a gain of only -18 dBd. A larger loop should

have improved gain.

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-30 dB

Loop Antenna (434 MHz)

x

2mm width

Loop: 433.9 MHz

12

50mm

37mm

Variable

Capacitor



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Semi-Loop

This is an unusual design that looks like a loop, but

requires no direct grounding. It is comparable to a loop

in performance, but requires no matching components.This antenna uses a trace that runs all the way around the

edge of a small pc board. The far (open) end is

capacitively coupled, through the board, back to the

transmitter end of the antenna. The antenna is tuned by

varying the length of the short overlapping line. Tuning

is not very critical. Hand effects will improve the

impedance, with little effect on tuning. Polarization is

parallel to the pc board, and the pattern is omni-

directional. Our design had a gain of -15 dBd at 433.9

Mhz. This design works very well for handheld devices.

As with any other design, the antenna should not run too

close to ground. For this design, the transmitter and othercircuitry, including battery, should be grouped around the

center of the board, leaving the antenna in the clear. The

circumference of the board needs to be well under one-

quarter wavelength. We have had good results with a

circumference of about 0.15 wavelength, and a linewidth

of 1 to 1.5 mm, when used in the 400 Mhz region. If the

design is used on a thinner board, the 5mm overlap will

need to be shortened.

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Semi-loop Antenna

434 MHz

5mm

42

35mm

x

Semi-Loop: 433.9 MHz

.060 inch thick FR4



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A modified Dipole Antenna

A Dipole can be shortened somewhat by bending the wire

or line back on itself, but not too close to itself. We built

a version on a printed circuit board, shown at right. This

antenna has the same performance as a full size dipole,but is more compact. The thickness and dielectric

constant of the board will affect the tuning, so the length

may need to be adjusted.

This type of antenna is an attractive solution where space

allows. However, a dipole should not be located close to a

large metal area or groundplane. The groundplane will

become part of the antenna, and performance will suffer.

Like the normal dipole, the radiation pattern shows deep

nulls and good gain. The impedance is a little lower, but

still near 50 Ohms. Like many of the previous antennas,

radiation from the face of the board is just as strong asfrom the long edge.

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147mm

55mm

26

27

3mm

line

width

Folded

Dipole:

433.9 MHz

x

feedpoint

Folded Dipole Antenna (434 MHz)

0 dB

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-30 dB



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The Slot

Common in radar and aircraft, a variation of the slot may have potential above 800 Mhz. A quarter-wave

slot cut into a metal area, if enough area is available, can provide omnidirectional coverage. Our sample

antenna at 916 Mhz required a 75mm long pc board. The length of the slot was cut to 59.5mm for 0.060

inch (1.5mm) thick FR4. A different thickness or dielectric will require changing the length of the slot.One end of the slot must be left open. The slot was fed near the closed end, in this case 4mm from the end.

The feedpoint impedance can be adjusted by moving the feed toward or away from the closed end. Tuning

was somewhat critical.

When the board is horizontal, the pattern is omnidirectional around the edge of the board, thus

horizontally polarized. We also see omni-coverage when the board is vertical (with the slot horizontal). In

this case, polarization is vertical! It may not make sense, but a horizontal slot is equivalent to a vertical

whip in this case. Gain is -4.5 to -6 dBd. The feed can be a trace on the backside of the board, with a via

used to make connection with the top of the board near the slot.

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x2mm wide slot

59.5mm long

4

25

75mmOpen Slot:

916.5 MHz

Half Open Slot Antenna

(916.5 MHz)

0 dB

-10 dB

or



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The Patch

The Patch antenna is a very low profile design, which may be a round or rectangular patch of metal very

close to a groundplane. It is usually printed on a circuit board and could be made as part of the enclosure.

Coverage is in any direction above the groundplane, a hemispherical area. The antenna does require a fair

amount of area on a board, which makes it more practical above 800 Mhz. It has a narrow bandwidth so

care must be taken to tune the size of the patch carefully. It is most sensitive to the thickness and dielectricconstant of the board and small variations will mistune the patch completely. It is also sensitive to

coatings, but not extremely sensitive to hand effects.

A practical example for 916 Mhz could fit into an area only 30 by 40mm. The patch size is 27mm wide by

38mm long for a board thickness of 0.060 inch. Thinner board or higher dielectric can require cutting the

antenna a little shorter. About one-tenth of an inch of board space should be left around any ungrounded

edge of the patch. One edge of the patch should be grounded with multiple vias through the board. The

antenna can be fed with a line crossing through the grounded edge to the 50 Ohm point on the patch, or

by a transmission line coming up through the bottom of the pc board. The 50 Ohm point is about 13mm

away from ground on our example patch. The 50 Ohm point for any design can be found by moving the

feedpoint toward or away from the grounded edge. The farther the feed is away from the ground vias, the

higher the impedance will be.

This type of patch is not a full-size, half-wavelength patch, so performance is not as good as it could be

with a larger size patch. A full-size patch has no grounded edge, so vias are not required. Our example

rectangular patch has a gain of -8 dBd. Placing the board against a larger sheet of metal will improve the

gain by another 4 dB. If the antenna is made wider than one inch, up to about 3 inches wide, a few more

dB can be gained. Polarization is perpendicular to the grounded edge. Gain is good in almost any

direction where the patch can be seen, but can drop rapidly when looking at the edge of the board.

The Trapazoidal version allows for less length so that it can fit into smaller spaces. Patterns and behavior

are the same, but the gain is a little lower. We measured about -12 dBd maximum, on a 40 by 90mm

board.

x

13

38

27

Rectangle Patch: 916.5 MHz

90mm

50mm

Vias to

backside2mm

wide

x

circuit

area

Vias to

ground

25mm

32

Trapazoidal Patch:

916.5 MHz



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Trapazoidal Patch over a

Small Ground Plane(916.5 MHz)

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-30 dB

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-30 dB

Trapazoidal Patch over a

Large Ground Plane

(916.5 MHz)



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Enclosures

An antenna should not be located inside a conductive, or metal enclosure. Care should be taken to keep an

antenna away from metal. If the conductive area is large in terms of wavelength (one half wave or more),

it could act as a reflector and cause the antenna to not radiate in some directions. If a metal box is used for

an enclosure, an external antenna is required.

Testing and Tuning

Antennas may seem to be a mystical art. Unlike many electronic devices, any change in nearby materials

or dimensions can affect antenna performance. Trying to build a published design does not guarantee

results. Testing an antenna design is necessary, tuning is usually required, and there are pitfalls along the

way.

A Network Analyzer is normally used to test the impedance or VSWR of the antenna. Some antennas that

have an impedance near 50 Ohms can be tuned by looking at a return loss or VSWR display. Low

impedance antennas may require the use of a Smith Chart display to get good results. In this case, the

antenna should be tuned to a point near the resistance line.

There are other options, such as a Spectrum Analyzer with a Tracking Generator, that can be used with a

directional coupler. The coupler will feed power to the antenna while feeding the reflected power from the

antenna back to the Analyzer. The coupler must have an isolation between the Generator and RF Input

Port of 20 dB or more. Calibration is done by noting the power readings with a 50 Ohm load connected

and then unconnected. Using this technique, Return Loss can be measured. If the antenna is near 50

Ohms, the Return Loss back to the RF Input Port will be high, due to the antenna absorbing most of the

power. A good antenna will show as a dip on the screen at the correct frequency. A dip of only 3 or 4 dB

(about a 5:1 VSWR) is normal for a low impedance antenna measured on a 50 Ohm Analyzer. A dip of 9

dB (about 2:1) or more indicates a well matched antenna in a 50 Ohm system. If the dip is not centered at

the right frequency, the antenna length or tuning needs to be adjusted.

Antenna measurements of any kind are tricky since the antenna is affected by nearby objects, including

the size and shape of the circuit board, and even by the cable connections to the Network Analyzer. Pass

your hand close to the antenna and the dip should move around a little. If it does not, the antenna may not

be connected properly. Antennas that are ground plane sensitive may see all additional wires as an

extension of that ground. Try wrapping your hand around the cable that goes to the Analyzer. If the

measurement changes much, you may need to try a different tactic. One possibility to minimize RF

currents on the cable is to put a few good high frequency toroids or some absorptive material over the

cable.

The best way to fine tune a remote transmitter antenna is by using the transmitter itself. Put an antenna on

a Spectrum Analyzer and try to keep other large metal objects out of the way. Find a place to locate the

transmitter that is away from metal and a few feet away from the analyzer. Always locate the transmitter

in the exact same spot when testing. If you have a desk that is wood, mark its position with a pencil ortape. If handheld, hold it in your hand just above the marking on the desk. Be sure to position your hand,

and the rest of your body, the same way during each test. Take a reading of the power level, and tune the

antenna to achieve maximum radiated power. The same thing can be done for a receiver. Transmit a

signal to it, and adjust the antenna to receive the lowest signal level from the generator.



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Common problems with antennas usually involve insufficient free space around the antenna. The antenna

cannot run close to ground or any other trace without effecting the antenna performance. This includes

traces on the other side of the board, batteries, or any other metal object.

Receiver performance can be degraded by digital circuits. Digital switching is very fast and creates high

frequency noise that can cause interference. Keep receiving antennas away from digital circuit traces. Tryto keep digital traces short, and run them over a groundplane to help confine the electro-magnetic field

that is generated by the digital pulses. If an external antenna is used, then use a coaxial cable. A

transmission line for G-10 material that is .06 inch thick requires a trace width of a tenth of an inch , half

of that for a .03 inch thick board. This results in a 50 Ohm transmission line that will carry RF with

minimum loss and interference.

High static voltages may damage sensitive semiconductors or SAWs. For antennas other than the loop or

patch (which are grounded), we suggest placing an inductor between the antenna and ground to short out

the static voltages. For the 400 Mhz region, a value near 200 nH is a good choice. At 916 Mhz, a more

appropriate value may be 100 nH.

Acknowledgments

The author would like to thank John Anthes, Harry Boling, and Jeff Koch for their assistance in thepreparation of this paper.

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